This invention relates to a charge-transfer image sensor.
Widely utilized is a charge-transfer image sensor comprised of a semiconductor integrated circuit.
FIG. 1 schematically shows an example of a conventional image sensor. Reference numeral 12 shows a photosensitive section formed on the surface of a semiconductor substrate, not shown. The photosensitive section receives an external light signal to produce a charge packet. The charge packet is read onto a readout shift register 14 for transferring charge packets. A shift electrode 16 receives a proper external voltage pulse signal to control a transfer of charge packets from the photosensitive section 12 to the readout shift register 14. An output voltage signal of the readout shift register 14 is taken out from its output terminal OUT 18 and fed back through an amplifier 20 to the input side of the shift register 14 for subsequent readout. The output voltage signal is taken out a required number of times through a closed loop including the readout shift register-and amplifier 20. The output voltage signal is obtained each time it is circulated once in the closed loop. At the output section of the readout shift register the charge packet is converted to a voltage signal and at the input section of the readout shift register the voltage signal is converted to the charge packet. In this case, however, the conversion efficiency at the input and output sections of the readout shift register 14 is not actually ideal. For this reason, an output signal is deviated from its original one for each circulation in the closed loop. In consequence, the output signal will include a greater error as the readout of the same signal is repeated. It has therefore been difficult to process highly accurate signals.